One example of a current sensor of this type is a current sensor that has already been disclosed by the present applicant in Patent Document 1 indicated below. The current sensor in the cited document has the following basic configuration. The current sensor includes a magnetic core configured so as to be split into a fixed first magnetic core and a movable (openable) second core so that a wire to be measured can be passed through an internal void, and a magnetic shield case (composed of a fixed first magnetic shield case and a movable second magnetic shield case) that covers the magnetic core with the object of shutting out external magnetic fields (external noise). The fixed first magnetic core is open on one side so as to form an approximate U shape, and the movable second magnetic core is supported by the second magnetic shield case and a core opening/closing mechanism so as to be capable of opening and closing the open side of the first magnetic core. The fixed first magnetic core is also provided with a Hall element, which is a magneto-electric converting element, and a negative feedback coil.
The current sensor of the configuration described above functions in combination with an externally provided amplifier as a zero-flux (magnetic equilibrium) measuring apparatus (as one example, a current detecting apparatus). In more detail, this measuring apparatus has the amplifier connected as a voltage-to-current converter to the output of the Hall element and has the output of the amplifier connected to one end of the negative feedback coil. The amplifier generates a negative feedback current that is proportional to the output voltage of the Hall element and supplies the negative feedback current to the negative feedback coil. A detection resistor that detects the negative feedback current as a voltage is connected to the other end of the negative feedback coil.
Here, the negative feedback current is proportional to the magnitude (hereinafter “quantity”) of the magnetic flux generated in the magnetic core. That is, the negative feedback current is Proportional to the current flowing in the wire to be measured. Accordingly, this measuring apparatus makes it possible to measure the current flowing in the wire to be measured based on the voltage detected by the detection resistor.
However, as disclosed in Patent Document 1, this current sensor has the following problem to be solved. In a current sensor of this type, the magnetoresistance of a closed magnetic circuit constructed by the magnetic core greatly increases at the part with the negative feedback coil and the part (gap) where the Hall element is disposed. This means that with this current sensor, magnetic flux leaks from these parts toward the magnetic shield case.
With this current sensor, the current flowing in the wire to be measured induces magnetic flux not only in the magnetic core but also in the magnetic shield case (such magnetic flux is sometimes referred to as “induced magnetic flux”), and depending on the magnitudes of the induced magnetic flux and the leakage magnetic flux, magnetic saturation may occur for the magnetic shield case. Note that magnetic saturation hardly occurs for the magnetic core since negative feedback is applied as described above so that the internally generated magnetic flux is zero.
When the current value of the current flowing in the wire to be measured is large and magnetic saturation has occurred for the magnetic shield case, the magnitude (or “quantity”) of the magnetic flux inside the magnetic shield case will peak and as a result, the magnitude (or “quantity”) of magnetic flux inside the magnetic core will increase by a corresponding quantity. Here, with a zero-flux (magnetic equilibrium) measuring apparatus (current detecting apparatus), the negative feedback current is increased so that the magnetic flux generated in the negative feedback coil increases in keeping with this increase. Accordingly, for the measuring apparatus that uses this current sensor, there is a problem that in a state where the current flowing in the wire to be measured reaches a large current region where magnetic saturation occurs for the magnetic shield case, the detection sensitivity increases (that is, linearity of the detection sensitivity is lost).
Patent Document 1 discloses, as a configuration for improving the problem described above, a configuration where out of the magnetic shield case, the volume of a specified part of the magnetic core where the magneto-electric converter (magneto-electric converting element) and the negative feedback coil are not provided is made larger than other parts (more specifically, an extra magnetic body is stuck onto the magnetic shield case).